α,β-unsaturated acids, particularly acrylic acid and methacrylic acid, and the ester derivatives thereof are useful organic compounds in the chemical industry. These acids and esters are known to readily polymerize or co-polymerize to form homopolymers or copolymers. Often the polymerized acids are useful in applications such as superabsorbents, dispersants, flocculants, and thickeners. The polymerized ester derivatives are used in coatings (including latex paints), textiles, adhesives, plastics, fibers, and synthetic resins.
Because acrylic acid and its esters have long been valued commercially, many methods of production have been developed. One exemplary acrylic acid ester production process utilizes the reaction of acetylene with water and carbon monoxide or the reaction of an alcohol and carbon monoxide to yield the acrylate ester. Another conventional process involves the reaction of ketene (often obtained by the pyrolysis of acetone or acetic acid) with formaldehyde. These processes have become obsolete for economic, environmental, or other reasons.
Another acrylic acid production method utilizes the condensation of formaldehyde and acetic acid and/or carboxylic acid esters. This reaction is often conducted over a catalyst. For example, condensation catalysts consisting of mixed oxides of vanadium and phosphorus were investigated and described in M. Ai, J. Catal., 107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987). The acetic acid conversions in these reactions, however, may leave room for improvement.
Thus, the need exists for improved processes for producing acrylic acid, and for improved catalysts capable of providing high acetic acid conversions in the formation of acrylic acid.